1. Field of the Invention
The present invention relates generally to semiconductor operation methods and, more particularly, to methods for controlling verify levels in semiconductor memories.
2. Description of Related Art
Non-volatile semiconductor memory devices are designed to maintain programmed information even in the absence of electrical power. Non-volatile memories in common use today include read-only memories (ROMs) that typically are programmed to store a fixed bit pattern at the time of manufacture and that subsequently cannot be reprogrammed. Programmable read-only memories (PROMs) are a form of field-programmable memory devices that can be programmed once by a PROM programmer. Erasable programmable read-only memories (EPROMs) are programmable like PROMs, but can also be erased, for example, by exposure to ultraviolet light that places all bits in the memory to a known state (e.g., a logic ‘1’). Electrically erasable programmable read-only memories (EEPROMs) are similar to EPROMs except that individual stored bits may be erased electrically.
Nitride read-only memory (NROM) devices represent a relatively recent development in non-volatile memory technology. (The acronym “NROM” is a part of a combination trademark of Saifun Semiconductors Ltd. of Netanya, Israel.) These devices store information in the form of localized trapped charges and may be referred to as localized trapped-charge ROM devices or cells. Some localized trapped-charge ROM devices are capable of storing multiple bits per cell. According to a typical implementation, charge can be stored in two regions of a charge-trapping layer that forms part of a typical localized trapped-charge ROM cell. Further, a multiple-bit localized trapped-charge ROM cell may be formed of a single transistor, leading to densities of localized trapped-charge ROM arrays that are higher than those achieved by many other forms of non-volatile memories.
A multilevel non-volatile memory device, for example, a localized trapped-charge ROM cell, is programmed by applying a programming pulse to terminals of the device. The programming pulse has the effect of injecting charge into a charge-trapping layer of the device, thereby raising a threshold voltage, Vt. Some multilevel devices have two data regions in the charge-trapping layer that are able to store independent data values by raising the Vt in a localized area near a source or drain of a multilevel memory cell. When the Vt is able to take on two distinguishable values, then each data region is capable of storing one bit of information.
Programming a data region of a multilevel memory cell normally involves applying a programming pulse and then performing a read operation, known as a verify operation, that estimates the value of Vt achieved by the programming pulse. Depending upon the estimated value of Vt, a subsequent programming pulse may be applied. The process continues until a desired Vt value is achieved, at which time the data region is said to be programmed. When two data regions are involved, a separate Vt is programmed/estimated for each data region.
Several factors intervene to make the process just introduced more difficult than has been suggested. These factors include an over-program effect whereby the programmed Vt value is higher than desired. Over-programming is an undesirable situation that has a detrimental effect on the long-term operation of a multilevel memory cell. Alternatively, a hard-to-program effect can make it difficult to achieve a sufficiently large value of Vt in some cases. Additionally, some coupling inevitably may exist between two regions in a multilevel memory cell. As a result, programming one data region can have an undesired effect on the programming of the other data region in the same cell, a phenomenon that has been referred to as a second-bit effect. Each of the above factors can act to reduce the utility of multilevel memory device architectures.
A need thus exists in the prior art to provide a method of programming multilevel memory cells that may reduce the undesirable effects of over-programming and hard-to-program problems. A further need exists for a way of reducing undesired aspects of the second-bit effect.